Dispute over life in Antarctic lake
CONTROVERSY has erupted over Lake Vostok, one of Earth's last unexplored frontiers, which lies deep under the Antarctic ice. Last week a team of Russian and French scientists claimed the lake is sterile. But American scientists insist that it is a potential source of undiscovered life forms, and are worried that Russian plans to drill right through the ice will contaminate it.
At the heart of the dispute is a small but diverse group of microbes found in the single core that has been drilled from the ice above the lake. The Russians and French say these are contaminants from the drilling and testing of samples in the labs. They also argue that the lake itself is too toxic to sustain life because of its extremely high levels of oxygen. If they are right, it would be the first lifeless water body found on Earth. As such, it could help us hone our techniques for the search for life under the polar ice caps on Mars, and in the oceans under the frozen surface of Jupiter's moon Europa. Lake Vostok hit the headlines in 1996 when its extraordinary size first came to light. Spanning some 14,000 square kilometres, and reaching a depth of nearly 800 metres in parts, it has been shut away from sunlight and the atmosphere for at least 15 million years.
There has been much speculation about the exotic ecosystems that might be found in this unique environment. French, Russian and American scientists looking for evidence of climate change began drilling through the ice sheet above the lake in 1989. But as they were not looking for life forms, the equipment was not sterilised and the samples were never properly stored. The drilling stopped 130 metres from the lake's surface to avoid contaminating it with antifreeze and the drilling fluid, kerosene. At this depth the ice is refrozen lake water, known as accretion ice. Four groups of researchers have found organisms in samples of this ice, suggesting that the lake supports a small but thriving microbial ecosystem. But molecular biologist Sergey Bulat of the Petersburg Nuclear Physics Institute, Russia, has created a furore by questioning the accuracy of this research.
At last week's meeting of the Scientific Committee on Antarctic Research in Bremen, Germany, Bulat claimed that these microbes are contaminants. His team compiled a list of 80 microbes found in their lab and in the drill hole that could have contaminated the Vostok ice. All but three of the organisms discovered in the ice are on the list. "We find bacteria in the accretion ice, but can eliminate most of them using our contamination database," says Bulat. Other researchers say the decontamination techniques used on the core sample, which involve washing it and removing the outer layers, would be completely effective. Brent Christner of the Montana State University in Bozeman has even tested for kerosene and other markers to distinguish the outside from the inside of the core. He is confident the cells he has found are authentic lake microbes. "The numbers of microbes in the accretion ice are two to seven times as high as numbers in the overlying glacial ice," says Christner. "This indicates that the lake is a source of life. All the data points to the fact that there are microbes and they're alive." But Bulat disagrees. "We don't have different results, only a different interpretation," he says.
Others are impressed with Bulat's methods, but argue he is being too stringent. "His methods are so severe they might cut out things that could be in the lake," says Chris McKay, at NASA's Ames Research Center in Moffett Field, California. Yet others are reserving judgement on the issue. "Of course the ice core is contaminated," says David Karl of the University of Hawaii in Honolulu, whose 1999 analysis of Vostok accretion ice reported up to 300 cells per millilitre of ice (Science, vol 286, p 2144). "We dealt with that potential problem in our processing of the core," he says. "I stand by the data that I published, but I have an open mind and would certainly capitulate if and when I am proven wrong."
The disagreements don't end there. Bulat and colleagues believe there are no bacteria in the ice because the lake is too toxic to support life. If so, Lake Vostok will be the only known sterile body of water on Earth. Though the lake itself has not been sampled, theoretical calculations predict that it has been pumped full of oxygen released from air bubbles in the overlying ice. None can escape, so oxygen concentrations are thought to have rocketed to around 50 times the norm for lakes. "Oxygen is very toxic at these concentrations," says Bulat. Added to this, he says, the probable concentrations of other by-products of this hyper-oxygenated environment, such as hydrogen peroxide and highly reactive free radicals, would destroy living organisms.
Jean Robert Petit of the Laboratory of Glaciology and Geophysics of the Environment at Grenoble, France, agrees. "It's a cold hell," he says. "It was once an open pond which probably contained life. Between 15 and 30 million years ago it started to freeze over, then gases accumulated in the lake and finally it sterilised itself," he says. This hypothesis has provoked scorn from John Priscu of Montana State University in Bozeman, author of one of the original papers on microbes in the Vostok accretion ice (Science, vol 286, p 2141). Priscu questions the assumption that the lake is poisonous, pointing out that Lake Vostok contains only around 10 times as much oxygen as other under-ice lakes in a region of Antarctica called the Dry Valleys, which are full of life.
"Free radicals are caused by radiation and ultraviolet light, and neither exists down there," he says. Besides, microbes can protect themselves against the hyper-oxygenation by producing antioxidants. "We should look for bio-signatures of microbes that can produce these," says Priscu. He believes the bottom of Lake Vostok, like that of many lakes, could be anaerobic and so provide a refuge from high oxygen levels in the upper parts. Other researchers also think that microbes will cope with Vostok's challenging environment.
"Almost every place we look, we find new types of organisms," says Scott Rogers from Bowling Green State University in Ohio, who has also analysed Vostok accretion ice and found very low numbers of what he thinks could be genuine bacteria. "I expect the same in Vostok." But what if the Russians are right? "It would be bigger news if Lake Vostok were found to be sterile, than the more probable result that it contains a small population of slow-growing microbes," says Karl. "We will not know the answer to this until we have a whole lake sample in our hands for proper investigation." This could happen soon.
A Russian team aims to break into the lake in the southern hemisphere summer of 2006-2007. Ignoring a proposal from an international panel to drill a new, cleaner hole through the ice and enter the lake using a self-sterilising "cryobot", the Russians plan to enter the lake from the existing, highly contaminated drill hole. They calculate that high pressure in the lake will prevent drilling fluid from polluting the water. Christner, among others, is concerned.
"The Vostok ice was not drilled under the cleanest of conditions," he says. "It is imperative that we employ methods to verify the authenticity of samples we obtain."
But with a complete lack of consensus on how to distinguish between authentic microbes and contaminant organisms, analysis of the lake water could prove as controversial as that of the ice above it. This has implications for the search for life outside Earth, because the lake is a unique test area for exploration of icy moons and planets. Considering that hundreds of microbes were found even in the clean rooms of NASA's Jet Propulsion Laboratory, which built the Mars Rovers that are now roaming the Red Planet, the arguments about contamination raised by Lake Vostok look set to continue.
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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